Sound reproducer



July 22, 1930. v. A. SCHLENKER souND REPRODUCER 4 Sheets-Sheet Filed Feb. 6, 1928 o \J u 1 Sywemtofl VESPER ANDERSON SCHLENKER dtlomw J July 22, 1930. v. A. SCHLENKER SOUND REPRODUCER Filed Feb. 6, 1928 4 Sheets-Sheet Svweufioz VEfiPER ANDERSON SCHLENKER 111.115, Clbtouwq k el July 22, 1930. v. .A. SCHLENKER 1,771,318

SOUND REPRODUCER I Filed Feb. 6, 1928 4 Sheets-Sheet 5 1 1 9. 1.3 as g4 3& Hus. abtovvwl Patented July vnsrnn'annnnson or omen, NEW JERSEY o,

sotnm mrnonucmz Application filed February 8, 1988. Serial No. 252,194.

This invention relates to the art of sound reproduction, and more particularly to a method and apparatus for converting electrical currents into sound waves, and has particular application to the reproduction of sound from electrical currents derived from received radio or similarsignals, or from mechanical records thereof, as in electrical phonographs. It has been customary in'the ast, for the purpose of reproducing sounds om electrical currents, such as those flowing in the output of an audio amplifier, to transform such currents into mechanical vibrations of a relatively small diaphra vibrations acoustica y by means of horns, trumpets, and similar devices. While a high qualit of sound reproduction is possible with such devices, nevertheless, the apparatus re- I quired is relatively bulky and expensiveto housein a pleasing and artistic manner, and expensive to ship. In particular, if it is desired to reproduce faithfully the relatively lower frequencies, as the lower pitch notes of the pipe organ, the horn required in such apparatus is of such excessive size as practically to prevent its utilization by the average person in the home.

It has also been pro osed to utilize diaphragms radiating to t e open air, such for example as cones and the like, and generally s aking, the quality of reproduction obtainable from such devices may be made to equal or exceed the quality obtainable from horns occupying substantiamhe same However, even with the types of suc direct radiatingdevices, the quality of reproduction obtainable is far from satisfactory as can be observed by a direct comparison between the original sound to be re roduced and the reproduction thereof obtaina 1e from such a device. 7

It is an object-of this invention to provide a method and apparatus which will be capable 5 of reproducing sounds with a fidelity closely approaching the original aoundsto be reproduced, and with a volume substantially corresponding thereto; his a further object of my invention to provide a direct radiating diaphragm which and to amplify such ace.-

will occupy a relatively small space, but which will, at the same time, be capable of deliver-' ing with a volume and quality closely approaching the originalsound over the ent1re audiblespectrum, and more particularly at the high and low ends of the spectrum.

It is a further object of my invention to provide a vibratile diaphra so arranged that although of finite size, it operates mechanica'lly and. acoustically as though it were of in- 4 It is a further object of my invention to provide a vibratile diaphragm so arranged that its reaction back upon the driving motor is substantially zero within the range of frebe characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its fundamental principles and-as to its particular'applications will ,best be understood by reference to the specification and accompanying drawing, in which:-

Figure 1 1s an elevation of the rear side of a sound reproducer according to my invention.

urel.

Figures 3 and 4 are detail sectional views of alternate arrangements for clamping the diaphragm in theframe.

Figure 5 is a sectional view of the driving coil. v

' Figure 6 is .an elevation of the rear side of one form of frame with'the damping unit holders removed.

' Figures 7 and 8 are sectionsthereof on lines 7- 7 and 88 respectively.

Figure 9 is a diagram showing the operation of my diaphragm.

Figures 10 and 11 are respectively sectional and front elevation views of the drivjing motor, Figure 10 being a section on lines 101O of'Figure 11.

Figure '12 is'a detail sectional view of the driving motor showing one method of bring- Y '80 Figure 2 is a section on lines 2-2 of Figphragm, the diaphragm and driving motor being so arranged that reflected waves do not react back upon the driving motor. As compared with the diaphragms usually used in horn t'ype reproducers, the diaphragm is of relatively large size. The size, however, is

,. of minor importance, but it should be understood that generally speaking, the smaller the diaphragm the less Will be the radiation of the diaphragm at low frequencies: that is to say, the low frequency cut ofi point rises as the diaphragm is made smaller.

I have found that satisfactory results may be obtained by the use of a clamped edge diaphragm approximately 24: on each side and in the general form of a square, modified as hereafter described; in fact, I have found that a diaphragm of such size when arranged ticularly satisfactory results are obtained and operated in the manner which .will hereafter he described in detail, will give a quantity and quality of sound reproduction far superior to anything now obtainable on the market, radiating at full efiiciency from about cycles to over 5,000 cycles, with a substantially fiat response characteristic over the entire included band of frequencies.

In accordance with one form of my invention, I provide a flat diaphra thin and light'material, capa le of sustaining considerable tension. I have found that a very satisfactory material for such a diaphragm is metal, although other materials, such as plywood, may be used, accordin to another embodiment of my invention. ar-

from aluminum alloys rolled in the form of a sheet. having a thickness of approximately .002 of an inch. As such materials may be obtained on the market and constitute per se no part of my invention, it is not believed necessary to describe them in-detail.

The vibratile material of the diaphragm is mounted upon a supporting frame consisting of a pair of clamping members extending around the periphery of the diaphragm, and the diaphragm is arranged to be held in position with a tension in the material of about forty pounds per inch in the case of aluminum alloys referred to. The tension of the diaphragm, it should be understood, must be at all times maintained within the elastic limit of the material used, as

of extremely otherwise the'material will be stretched and permanently deformed, which will seriously affect the reproduction. The tension should be made as great as possible for the purpose of raising the speed of travel of flexural =vibrations in the material, since the efficiency of adirect radiator depends upon the degree of coupling between the air and the diaphragm, which in turn is a function of the ratio of the velocity of propagation of waves in the diaphragm to that of waves in the air. Mathematically, the optimum radiation would be obtained in case this ratio were made equal to unity, by stretching the diaphragm sufliciently. However, it is evident that the elastic limit ofthe material of the diaphragm limits the amount of tension which can safely be used.

In accordance with another form of my invention, I may use an acoustically free edge diaphragm made up of material which is so chosen 'as to have considerable stiffness together with minimum mass. Satisfactory results have been obtained by the use of a plywood diaphragm madeup of thin layers of balsa wood, glued together, with thegrain crossed. Such a diaphragm has considerable stiffness and small mass, andI have found that satisfactory results may be obtained without applying tension to such a diaphragm, the inherent stiffness and light weight of the material contributing to afford a velocity of wave propagation in the material approaching that obtainable in the tensioned diaphragms already described.

Such a diaphragm may be operated as an acoustically free edge diaphragm affording certain advantages: for example, while the amplitude of motion at the periphery of a clamped diaphragm is zero, the amplitude of motion at the periphery of an acoustically free edge diaphragm is double the amplitude obtained at a point within the diaphragm, and this larger amplitude of vibration obtainable at the edge of such a diaphragm makes it possible to utilize a diaphra of somewhat smaller size, without sacr1ficing efficiency of radiation at the lower frequencies, provided precautions. are taken to prevent the surging of air around the edge of the diaphragm. This may be done by sus pending the diaphragm at its periphery from a hinge of thin linen, silk or similar material having a sufiiciently closely woven texture to hinder the surging of air through it, and attaching the outer edge of this hinge to a suitable bafile board or to the interior wall of a cabinet? It has already been stated that very satisfactory results are obtainable from a tensioned diaphragm clamped at the edge, having the general form of a square 24 on a side: comparable results may be obtained by the use of a free edge diaphragm, such as mil lthe material of the diap ragm, which travel outward toward the periphery: upon reach-. ing the periphery, such waves are reflected inwardly 180 out of phase in thecase of a clamped'edge, and in phase in'the case of a free edge, where they interact with other waves travelling in other directions in the diaphragm. This interaction, for various reasons, I believe to be a prominent cause of distortion in the sounds reproduced by such a diaphragm, since it may cause nonuniform radiation of sound at various fre-' quencies, accentuating certain frequencies and diminishing others, and reacting back on the drive in a-complex manner.

In accordance with the preferred embodiment of my invention, I shape the periphery of the diaphragm and propagate waves therein in such a manner that after reflection at the periphery, the reflected waves present a substantlally straight line wave front, and

I then absorb a large part ofthe energy of such reflected waves after they have'travelled inwardly from the periphery of the diaphragm a suflicient distance to reach a point at which theyv would interfere or react with other waves reflected from other parts of the periphery of the diaphragm. In general I obtain this result by arranging the periph- I of the diaphragm in the shape of a para ola, and by impressing the vibrations upon the diaphragm at or near the local point of the parabola in such a manner that the vibration waves travel in a direction radiall outward from the focus. As a re sult, a er reflection. the wave front of the reflected wave will be a straight line, perpendicular to the axis of the parabola and travelling in the, direction of the. axis, away of a parabolic reflector.

intersection half wa and extends inwardly su from the focus, in a manner analogous toIthe reflection of light from asource at the focus I preferabl arrange damping material upon the diaphra m in a line parallel to the wave front of reflected waves, whereby each reflected wave is simultanenously damped out and substantially absorbed by the damping material over its entire front. Preferably, the diaphra is arranged to have the general shape 0 a s uare, the four sides of which, however, instea of being straight lines, are )arabolic and curve inwardly to a,point of between the opposite corners. The dampin material is preferably, but not necessari y,-arranged in a line coinciding with the diagpnals of the square ciently far to cover the wave front of the waves reflected from rovides what I term a one-way transfer 0 energy from the drive to the diaphragm; that is to say, since reflected waves never return to the propagation of new waves, which is a serious defect of all prior direct radiating diaphragms.

The damping material may be regarded as a mechanical resistance analogous to the re sistance in an electrical circuit, and it is apparent that if complete absorption is to be obtained", the mechanical resistance of the damping material should be equal to the surge impedance of the diaphragm, measured at the particular point atwhich the damping material is applied. When this condition is obtained, or closely approximated, mechanically the diaphragm operates with respect to the drive as though it were infinite in size; that is to say, the reflected waves are terminated at the damping material and are not reflected backward from the end point, as the -would be from the boundary of a finite iaphragm. If such a diaphragm may be conceived as operating in a complete vacuum, the diaphragm may then be regarded as a mechanical power transmission line, in which power is delivered from the driving unit through the diaphragm to the damping material, where it is dissipated as heat. When the diaphragm is submerged in an energy absorbing medium, such as air, it will be understood that not all of the energy delivered to the diaphragm by the driving mechanism, is delivered to the dampin material: in fact, if this condition o'btaine the efliciency of the-diaphragm would be zero; because of the air loading upon'the diaphragm, compression or rarefaction waves are set up in the air adjacent the diaphragm, absorbing from the diaphragm a certain proportion of the energy. It will be understood that it may be permissible to depart from the mathematically exact condition in which the vibrations are completely terminated by the damping material, since a certain proportion of the energy may be allowed to pass along the diaphragm across the damping materialwithout seriously affecting the results.

I have found that such a sound reproducer small and light cylindrical coil is vibrated axially in an annular air gap traversed by a constant uni-directional magnetic flux. By supplying the electrical currents which it is 'desired to convert into sound to the vibratthat distortion in the radio recelver or the ing coil of such a device, the coil is caused to vibrate axially with an amplitude and frequency very closely approaching the amplitude and frequency. of the driving currents, and the distortion inherent in all known types of magnetic armature driving motors spring biased to neutral position, and thereby hav ing inherent elasticity is eliminated.

The apparatus thus far described is caable of such faithfulness of reproduction,

' and the audio amplifier utilized for supplying crossbrace 4; to corner B.

it, should be of high quality and should not be operated beyond the limits of distortionless operation, as otherwise rattling and accentuation of high frequencies appear, which, in the past, have not been particularly harmful or noticeable, since they have been masked by greater distortion in the reproduccr itself.

Referring now more particularly to Figure 1, 1 designates generically a frame or peripheral clamping ring provided with a plurality of closely spaced holes 32 for the passage of clamping bolts. It will be understood that two such clamping rings 1 and 1 are utilized to engage and hold the diaphragm 30 tightly between them. For the purpose of supporting the driving motor and the damping units, one of the clamping frame members may be provided with cross braces 2, 3, 4 and 5, and also with inwardly extending braces 6, 7, 8 and 9, which extend inwardly to motor supporting ring 10. For the purpose of securing the damping units in position, I may provide a pair of parallel braces 16 and 17 extending from cross brace 5 tocorner C, which may be formed integral with the frame or as a separate frame and bolted thereto as shown. Similar braces 18 and 19 extend from cross brace 2 tocorner D, braces 20 and 21 from cross brace I 3 to corner A, and braces 22 and 23 from Inwardly extending projections may be mounted upon the cross braces 2, 3, 4: and 5, to which the damping unit frames may be fastened to hold the damping material in position, and a handle 11 may be provided. Naturally, sufficient clearance must be allowed between the inner faces of these braces to permit vibration of the diaphragm, without the possibility of the diaphragm striking the-braces. This will preferably he It will be understood that the internal bracing portions are preferably provided only on one of the clamping frames, since these braces are primaril for the purpose of holding the motor and amping units in position,

and not necessarily for insuring structural ri idity of the rim 1. %lach frame member or clamping ring is referably made in one piece, and preferably ilom the same material as the diaphragm in order to insure against changes in the tension of-the diaphra m as a result of temperature variations. The holes in the ring for the clamping bolts, are preferably sufiiciently close together 'so that a uniform tension is exerted around the entire peri hery of the diaphragm, and the number an spacing of the bolts may be determined by experiment. It will be understood that in general the heavier and more massive the frame, the fewer the number of bolts necessary. Preferably, the holes in one of the frames will be tapped to receive bolts, but it will be understood that as an alternative, clamping nuts may be utilized if it is not desired to tap the frames, or, inte ral lugs 60 or rivets 60 may be convenient y provided in one of the frame rings, such as 1', if the rin is cast, which may engage-corms ondin oles in the other frame ring 1", an 7 may be upset or hammered to form a head, maintaining the frames 1 and 1" in position and thereby securing the diaphragm 30, which is heated or mechanically stretched to insure the predetermined tension.

Damping material 42, such as lamb -wool, felt, and the like, is preferably provided in the space between braces 20 and 21, 22 and 23, 16 and 17, and 18 and 19. Various damping materials may be utilized but preferably utilize a layer of lambswo0l next to the diaphragm 30, which is backed by a layer of felt.

The diaphragm is driven preferably by means of an annular coil 34, which is traversed by the electrical currents which it is desired to convert into sound. The annular coil 34 moves axially in an annular air gap in magnet 37, which comprises a center pole piece 38'and a separate outer pole piece 40 carried by a spacing ring 41.

The driving coil may be mounted upon a ring 35 of conducting material, such as aluminum or copper, provided with a flange 36 at one end, by means of which the coil may cemented to the diaphragm 30. The ring will preferably be split axially at one point to prevent its acting as a short circuited turn,

but in case a filtering action is desired to su press high frequency harmonics generated y tube overload, it maybe in the form of a closed ring. The eddy current losses will then operate to dissipate a certain proportion of the energy of the higher frequencies, and

the result is a suppression of high frequencies, so that distortion due to tube overload is decreased, and the use of a low-pass filter 1s rendered unnecessary. The coil will be made as rigid and light as possible, and preferably Considerable care must be'taken inbringincgs consists of a small number of turns with an optimum space factor, so that the resistance is kept extremely low. In general, it may be stated that the coil assembly will be made as light as possible consistent with sufiicient structural rigidity to maintain its shape in operation, thereby permitting the use of a relatively small air gap in which there may be Obtained a high density of ma netic flux. The particular arrangement of t e drivin coil and magnet is shown in Figures 10 an 11, in which 37 designates generically a core and inner pole piece formed in what may be termed a'eup with an internal, projection, the internal projection 38 forming the inner pole face, and the outer portion or rim of the cup 39 resting on and making close magnetic contact with the outer pole piece 40'carried .on spacing member 41 of brass or other suitable material. The outer pole piece 40 is in the form of a ring, provided with cut out portions 45 and spaced 90 from each other to ,permit the spacin ring 41 to rest upon internally projecting ugs 12, 13, 14 and 15 in motor sup-' portin ring 10. The outer pole piece 40 is secure to the spacing ring 41 by. means of rivets 60. 61, 62 and 63, and the spacing ring is provided with outwardly extending ears 65, 66. 67 and 68, which rest -u on and engage lugs 12, 13, 14 and 15, which ugs are preferably tapped to engage screws 46, securing the spacing ring and outer pole piece rigidly within the frame ring 10. The outer face of cars 65, 66, 67 and 68 preferably are arranged to make a snug fit with the inside of motor supporting ring 10, while ample clearance is provided between the outer periphery of outer pole piece 40 and the inside of motor supporting'ring 10. The outer rim portion 39 of the magnet is cut away, as at 45', to per- I mit access to the screws 46. It will be observed that this construction provides a cavity 43 within the center pole piece, which maybe utilized for the purpose ofhousmg an energizing winding in case the magnetic flux across the air gap is to bederived from an electromagnet, but it will be understood 3 that the entire pole piece 37 and outer pole piece 40 may both be permanent magnets.

The driving coil 34 is cemented or otherwise secured to the diaphragm for axial movement in the annular gap between the inner pole piece 38 and the outerpole piece 40. The outer pole piece 40 is prefera 1y provided with a tapered edge 48, serving-to concentrate the magnetic flux lines in a small area, and the cross sectional area of the magnetic path at any. ooint willpreferably be made about four times the cross sectional area of the flux path the air gap. The magnet is preferably held in place by means of metal straps 26 and 27.

ures 12 and 12, I have s own one method f Referringnow more articularly to Figsupplying currents to the driving coil 34.

vent the leads chattering against other parts of the structure during operation. For the purpose of making contact to the coil, I may provide a strip 71 of relativel light and wide ribbon, such as silk, and I no or otherwise and 7 3 preferably of phosphor bronze, approximately .001 of an inch thick andabout wide. 'One' end of the ribbon71, with the conductor secured thereto, is cemented or glued to the supporting ring adj aoent the connection to the diaphragm at the flange 36 The two ends of the coil 34 are soldered to the conductors 72 and 73 respectivel A hole 70 is provided in the outer pole ace and spacing ring 41 through which the ribbon 71 1s passed into the cavity 43 in the magnet 37.

The ribbon is then passed through one of the cut awayportions to a suitable terminal block 7 4 whence permanent leads may be taken ofi to the exterior of the reproducer in any suitable manner. The ribbon 71 is preferably glued to the outer-pole piece at point 75, leaving justsufiicient slack to permit free vibration of the diaphragm, after which the passage may be packed with damping material such as wool or felt 76. The use of a relatively wide ribbon presenting alarge sur face and having'a minimum mass, provides a relatively heavy air damping upon the lead structure and prevents the development of vibration of excessive amplitude therein, which would tend to stress the-leads beyond the elastic limit and result .in crystallization.

As an alternative, when a" metal diaphragmis used, it is possible to ground one.

end of the coil 34 upon the coil support 35 or diaphragm 30, and in this arrangement it is only necessary to provide one conducting secure thereto a pair of strips or ribbons 72..

strip upon the ribbon 71, the other side of the circuit being formed by the diaphragmand frame. Under certain conditions, one or both of the leads may be glued or cemented to the diaphragm itself and carried upon the diaphragm to a terminal-block on the frame.

Referring now more particularly to Figure 7 i 6, I have shown the frame with the damping unit supports 16, 17 18,. 19,20, 21, 22, and 23 removed: the lugs 16, 17', 18, 19 20, 21, 22 and 23", are provided on cross braces 2, 3, 4 and -5, and the damping unit frames 16-17, 18-19, 20 .21, and 22-23 carrying the damping material may be bolted between lugs 16 and 17 and corner C, lugs 18 and 19and corner D, lugs 20 and 21' and corner A and lugs22 and 23. and corner- B respectively.

- of a strip of light and closely woven linen or silk fabric having its inner edge glued or cemented to the diaphragm 80, and its outer edge similarly secured to an internal carrying plate 84 mounted within the walls 85 and 86, 87 and 88, of a cabinet. The supporting hinge 83 which supports the diaphragm is preferably very light and is preferably only wide enough so that the periphery of the dia phragm is axially unrestrained and vibrates in substantially the same manner as it would if entirely free at the edge, developing twice the amplitude of vibration at the periphery as within the diaphragm. Supporting plate 84: will preferably follow the contour of the diaphragm and extend as close as possible thereto, while allowing freedom of motion of the periphery of the diaphragm, so as to obtain a minimum area of hinge consistent with freedom of motion. As already stated, the large amplitude o vibration developed at the periphery of a free edge diaphragm permits the use of a smaller diaphragm without raising the low frequency cut off point, and such a construction may be preferred where it is desired to keep the size of the reproducer as small as possible.

It will now-be understood that with the apparatus so far described, if the field magnet is energized and a fluctuating current passed through the coil 34, this coil will be caused to vibrate axially and the vibrations will thus be communicated to the diaphragm; these vibrations will travel outwardly in the material of the diaphragm in a manner similar to the outward travel of ripples upon a mill pond when the surface is disturbed as by dropping a stone in the water.

If an ordinary circular diaphragm were used instead of the arrangement herein shown, when these vibrations reached the pe riphery they would be reflected from the boundary backwardly toward the center of the diaphragm. This reflection would result in interaction between the reflected waves returning from the periphery of the diaphragm and the original waves travelling toward the periphery, and would result in the production of standing waves, accentuating the radiation of certain" frequencies and diminishing'others, and would react on the motor by causing the diaphragm and driving coil to tend to takea position other than that which it would take if the diaphragm were in fact infinite. This will cause the mechanical impedance of the diaphragm to vary in a complex manner, seriously distorting the response characteristic: and in general, producing many undesired results, evident as various types of distortion and undesired noises. 4

Referring again to Figure 6, it is desired to point out that the curve formed by points D, H, E and B, is a paarabola; similarly, curves A, E, F and C; F, G and D;and C, G, H and A, are parabolas. These parabolas are preferably all plotted together from the same equation and all have a focus at point K, which is the center of the driving coil. It will be observed that the diaphragm is thus made up of a central square section, bounded by the vertices E, F, G and H, and that sur rounding this central section, there are four sections, A-EH, EBF, FCG, and GDH.-1 The inner boundaries of each of these outer sections, it is to be understood, are only imaginary boundaries. The other two boundaries are parabolic curves intersecting atthe points A, B, C, and D. It can be shown mathematically that the angles at the points A, B, C and D, formed by the intersection of two parabolic boundaries, are right angles, but for manufacturing purposes, I prefer to round off these corners slightly, as shown, rather than to preserve the'mathematical shape exactly to points A, B, C and D.

Turning now to Figure 9 in which I have shown diagrammatically the action of the diaphragm, it will be understood that vibrations impressed upon the diaphragm by the driving coil 34 cause flexural waves in the material, which travel outwardly in a direction outward from the focus K toward the periphery in the form of expanding circles, as represented by the series of concentric circles around point K.

Let us now consider the pathv followed by any particular point upon oneof these waves as it travels outwardly. It will be seen that the path followed is, a straight-line: for example, a path may beassumedfrom K to a point P, which is any point arbitrarily selected upon the boundary E -B. Impinging upon the boundary at point P, the wave will be reflected in accordance with the laws of reflection: in other words, it will be reflected at an angle of reflection which is equal to the angle of incidence, but since E-B is a section of a parabola whose focus is at K and whose axis is on the line AK, the path after reflection will beparallel to the axis If a similar point Q, be assumed upon the parabola B'-F, the reflection of a wave from .this point, will be parallel to the same axis but in the opposite direction from the path followed by the wave reflected from point P. It will be seen, therefore, that two waves lUU striking similar points upon the opposite boundaries EB and'F-B, are reflected toward each other in the same strai ht line,

travelling at the same speed and W111 inter-. sect on the axis KB. The same is true ofs points P1 and Q1, and P2 and Q2. It will be seen that all of these paths are parallel, and

since the distance K-P--O is equal to the distance KP1O1, which is also'equal to the distance KP2O2, it will be observed, that a circular wave travelling in a direction outwardly from the focus K, after reflection, possesses not a circular wave front but a straight line wave front, as indicated by the dotted lines parallel to the KB axis.

It is also apparent that interaction between twosetso reflected waves cannot-take place until one of these has passed across the axis KB; and if now a layer or line of damping material be laid along the axis KB in the path of the reflected waves in a manner to absorb and dissipate theenergy of such-reflected waves, the reflected waves will be substantially terminated at this point, and by proper control of the damping ma'-' terial so that the mechanical resistance introduced at such point is equal; to the mechanical surge impedance of the diaphragm measured at that point, no r'eflection will take place. A variation from the proper value of damping will cause reflection to take place at this point.

' It will be observed that along the boundary EF adjacent point B, the reflection is not in accordance with the laws already de} scribed, since the curve at this point departs from a true parabola, but it will be observed, that when a vibration wave arrives-at point B' from the source K, it is necessary to pass the entire line of damping material. It results from this that the reflection action at' corners A, B, C and D, for all practical purposes, may be disregarded. Since the entire diaphragm is symmetrical, an action takes place in the other quadrants similar to that alreadydescribed; for example, a-point on a ized, practical considerations of cost Wlll natwave travellin from K to point B or S on the boundary ,E is reflected inwardly in a direction perpendicular to the axis A- K,' and the'provision of damping material along the axis A-K in amanner similar to that already described. suifices to terminate this wave on the axis A-K; As a result of this arrangement; it will be seen that while the diaphragm is of finite size, it acts acoustically and mechanically as though it were of infinite size, in that waves emitted from the source K never return to thesource K bv reflection, and consequently do not react back upon the driving motor.

While I have referred to the damping' ma terial as being disposed upon the axis K,-B,

. it will be understood that while'this may be assumed mathematically, it is somewhat dif- WhileThave referred to the use o: t wool,-

felt, and similar. materials-1 for damping, it Will be under'stood'tha't other arrangements maybe utilized, such as air damping, which may be obtained under certain conditions simply by enclosing the air over a portion of the diaphragm, or electro-magnetic, or eddy current, damping, may be utilized by providing conductors attached to the diaphragm,

moving through. a magnetic field, or by the provision of magnets adjacent the diaphragm, in case it is a conductor, whereby vibrations thereof cause the generation of eddy currentsin the diaphragm itself. In all of these arrangements, as wilLbe understood,

the purpose and effect of the damping is to absorb the energy of vibration-to a sufficient extent so that standing waves arenot produced on the diaphragm and so that reflected waves are not reflected back tolthe driving motor, and while I prefer to apply .the dam 'ing material alongthe four (llagonals K K A-, KD, and K--C, extending outwardly to the boundary of the diaphragm and inwardly-tolines EF, '.H-E, GH and FG, one or more of the damping units may be omitted, those retainedbeing preferably in adjacent sections'rather than opposite section's'. Such omission, will, in general, be accompanied by distortion, due to passage of reflected wavesfback to. the drive, but a certain amount of'such reflection may be tolerated if the attenuation of the diaphragm is high or if the amplitude of vibration is kept small. 4

, While mathematically there is nolimit to {the size of the diaphragm which may be utilurally impose a-limit, beyond which it is not desirable togo. a

It willalso be understood that since the radiation of sound from a direct radiator,

. such-as described," isgoverened bylaws similar to those governingthe' radiation-of elecficult to obtain in practice; and preferably, lowed in travelling through-the air from'one 13o side of the diaphragm to the other. This may be accomplished by providing a baffle structed according to the principles herein disclosed, which is made approximately 24 on each side, will operate at full efficiency at a frequency as low as approximately 80 cycles, and will also operate at full eiflciency at frequencies as high as 5000 to 6000 cycles, the intervening range of frequencies being covered with a substantially flat and uniform characteristic in fact, I have found that such a reproducer is much more free from distortion than the other elements with which it is customarily operated, and whereas, with an ordinary type of cone loud speaker or horn such as is now obtainable on the market, distortions arising in the radio receiver, particularly those due to overloading Oftll tubes and batteries, are not evident to the lay observer, such distortions are at once evident even in as small amounts as those resulting from overload when a sound reproducer according to my invention is utilized. It is therefore desirable in operating a reproducer according to my invention, to take particular pains to prevent overloading of any of the tubes feeding it, and to make sure that batteries and other current supply devices are fully capable of carrying their rated load without loss of voltage, as otherwise rattling and accentuation of high frequencies will be evident, which might erroneously be attributed to the sound reproducer itself. While I have described the operation of my invention in the reproduction of sound from electrical energy, it will be apparent that a sound reproducer in accordance with my in vention is, to a certain extent, reversible, and may be utilized as a transmitter.

Also, while I have described certain pre ferred embodiments of my invention, it will be understood that modifications and changes,-

may be made without departing from the spirit and scope of my invent-ion,.as will be understood by those skilled in the art.

I claim i 1. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm adapted to be driven from a point within the periphery thereof, said periphery comprising a curve causing sound waves originating at said point to be reflected from said curve to present a straight line wave front after reflection.

V 2. A sound reproducer for voice and music comprising a relatively large-vibratile diaphragm adapted to. be driven from a point within the periphery thereof, said diaphragm comprisinga plurality of sections, the periphery of each section comprising a curve causing sound waves originating at said point to be reflected from said curve in parallel paths. 3. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a periphery, a portion of which is substantially in the form of a por tion of a parabolic curve.

A'sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a. periphery, a portion of which is substantially in the form of a portion of a parabolic curve and means for propagating vibration waves therein in a direction radially outward from the focus thereof.

5. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a periphery, a portion of which is substantially in the form of a portion of a parabolic curve and means for propagating vibration waves therein in a direction radially outward from the focus thereof, said driving means being substantially devoid of elasticity.

6. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm, the boundary thereof having the shape of intersecting parabolic curves.

7. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm, the boundary thereof having the shape of intersecting parabolic curves having a common focus.

8. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm, having a central section and a plurality of outwardly extending sections surrounding said central section,:said outwardly extending sections having parabolic boundaries.

9. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a central section, and a. plurality of outwardly extending sections sur-' rounding said central section, the boundaries of said outwardly extending sections being similar parabolic curves having a. common focus.

10. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a periphery, a portion of which is substantially in the form of a portion of a parabolic curve, means for propagating vibrations therein in a direction radially outward from the focus thereof, and damping material mounted thereon in a line parallel to the wave front of reflected waves.

11. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a periphery in the form of intersecting parabolic curves, meansfor propagating vibration waves therein in a direction radially outward from the foci thereof, and damping material mounted thereon in a line parallel to the wave front of reflected waves.

12. A sound reproducer for vcice and music comprising a relatively large vibratile diaphragm having a periphery in the form of intersecting (parabolic curves havin a common focus an also having a line maximum amplitude of vibration, means for ropagating vibration waves therein in a irection radially outward from said focus, and damping material mounted thereon along the line of maximum amplitude of vibration.

, waves therein in a direction radia ly outward from said focus, and damping material associated with a. plurality of said outwardly extending sections along the line of maximum amplitude of vibration.

14. A sound reproducer for voice and music comprising a relatively large vibratile diaphragm having a periphery in the form of intersecting parabolic curves having a common focus and having axes at right angles to provide a diaphragm having an axis of symmetry, means for propagat vibration waves therein in a direction radia y outward from said focus, and damping material associated therewith in the ath of reflected motion upon a relatively large vibratile diaphragm to produce travellin waves therein, reflecting said waves from t e various portions ofthe periphery of said diaphragm in such manner as to produce waves having astraight line front after reflection, permitting said reflected waves to'travel inwardly from said ri hery and absorbing the energy of sai re ected waves at a plurality of points on said diaphragm simultaneously to revent interaction between waves reflected rom different sections of said periphery.

18. The method of converting mechanical motion into sound waves corresponding thereto with a relatively large vibratile diaphragm having a eriphery in the form of intersecting parabo as having a common focus, which comprises applying said motion to said diaphragm in a manner to roduce there- I in vibration waves in said diap ra travelling in a direction radially outwar from said focus, reflecting said waves from the periphery of said diaphragm, permitting said reflected waves to travel inwardly from said periphery, and absorbing the energy of said reflected waves over the entire reflected wave front simultaneously to revent interaction between waves reflected m difl'erent portions of said riphery.

Signed at ew York city, in the count of New York and State of New York, this 28th day of January, A. D. 1928.

VESPER ANDERSON SCIILIIKIJ.

waves and along the axis 0 symmetry of said Q diaphragm. v v, 15. A sound re roducer for voice and music comprising a re atively large vibratile diaphragm having a periphery comprising a portion of a parabola, means for propagating vibration waves in a direction radially outward from the focus of said parabola, and damping material mounted on said diaphragm in a line perpendicular to the axis of the said parabola,

a 16. The method of converting mechanical vibrations into sound waves corresponding thereto, which com rises impressing said vibrations on a relatively large vibratile diaphragm at a int within e periphery to produce trave said diaphragm, various portions of theperiphery of said diawaves in the material said waves phragm in a manner to produce after reflection a plurality of strai ht line wave fronts,

permitting said reflec waves to travel inwardly from said periphery, and absorbing the energy of said reflected waves to prevent interaction between waves reflected from different portions of said periphery.

17. The method of converting mechanical vibrations into sound waves corresponding thereto, which comprises impressing said 

